1. Field of the Invention
The present invention relates to piezoelectric oscillators. More particularly, the invention relates to surface-mount type piezoelectric oscillators including quartz vibrators and providing reference frequencies used for mobile terminals.
2. Description of the Related Art
Conventionally, a surface-mounted-type quartz oscillator includes a quartz vibrator as a discrete packaged component. In the quartz oscillator, a quartz vibrator package containing a quartz member is disposed on a box-shaped circuit substrate having surface-mounted circuit components defining an oscillation circuit, a temperature-compensating circuit, and other such circuits. In addition, connection electrodes disposed on the upper surface of the circuit substrate are bonded via solder with electrodes disposed on the lower surface of the quartz vibrator package. With this arrangement, the quartz vibrator is electrically connected to the oscillation circuit, the temperature-compensating circuit, and other such circuits. Furthermore, the circuit substrate is mechanically coupled to the quartz vibrator package.
In the quartz oscillator, when the circuit components to be surface-mounted are disposed on the circuit substrate, first, the circuit components such as a transistor, a capacitor, and a resistor, are temporarily fixed to the circuit substrate by solder paste. In addition, solder paste is applied between the electrodes disposed on the upper surface of the circuit substrate and the electrodes on the lower surface of the quartz vibrator package to mount the quartz vibrator package on the circuit substrate. Then, the quartz oscillator is heated in a reflow furnace, in order to bond the circuit components and the quartz vibrator package to the circuit substrate by reflow soldering.
However, as described above, when the circuit components and the quartz vibrator package are mounted by reflow soldering, it is necessary to remove flux residue remaining after soldering because the flux residue tends to corrode a portion of the electrodes disposed on the circuit substrate and portions of the circuit components. As a result, functional problems including frequency variation are caused in the quartz oscillator.
In order to remove such flux residue, the quartz oscillator after soldering is washed with cleaning solution to dissolve the residue. As the cleaning solution, chlorofluoro hydrocarbon solvents used to be applied. However, recently, there has been concern about the depletion of the ozone layer due to chlorofluoro hydrocarbon. Thus, the use of water-based cleaning solutions and chlorofluoro hydrocarbon substitute solvents having an inferior cleaning effect has been necessitated. On the other hand, it is difficult for the water-based cleaning solutions and the chlorofluoro hydrocarbon substitute solvents, as alternatives to the chlorofluoro hydrocarbon solvents, to enter clearances between the circuit substrate and the quartz vibrator package. As a result, the flux residue cannot be sufficiently removed. In addition, such cleaning solutions or solvents are not easily discharged and often remain in the quartz oscillator after cleaning.
Meanwhile, in Japanese Unexamined Patent Application Publication No. 6-232527, small holes or slits are formed in a main surface of a printed circuit board to wash away the flux residue, and a chlorofluoro hydrocarbon substitute solvent can easily enter the small holes or the slits. However, it is very difficult to form such small holes or slits in the main surface of the circuit substrate since circuit components are densely packed on the main surface of the circuit board, which requires miniaturization while maintaining a high integration density. In addition, wiring patterns are arranged in a grid form. As a result, in order to form small holes or recesses in such a structure, the size of the circuit board must be increased.
In order to overcome the problems described above, preferred embodiments of the present invention provide a compact piezoelectric oscillator having a structure that allows a cleaning solution to reliably flow and spread through the inside portion of the piezoelectric oscillator and then be reliably discharged from the oscillator.
According to one preferred embodiment of the present invention, there is provided a piezoelectric oscillator including a circuit substrate, a cavity defined in the upper surface of the circuit substrate for mounting therein at least one circuit component to be mounted on the circuit substrate, a vibrator package containing a portion of piezoelectric member, the vibrator package arranged to be integrally mounted on the circuit substrate so as to cover the cavity, and a clearance defined in a wall surrounding the cavity of the circuit substrate to connect the inside of the cavity and the outside thereof.
In the above-described piezoelectric oscillator, since the wall surrounding the cavity has the clearance defined therein, a cleaning solution can easily enter the inside of the oscillator through the clearance of the circuit substrate. As a result, even when flux residue remains on the circuit substrate or between the circuit substrate and the vibrator package after soldering, the flux residue can be completely washed away. Moreover, the cleaning solution can easily be discharged through the clearance of the circuit substrate after the cleaning. With this arrangement, changes in the frequency of the piezoelectric oscillator occurring with the passage of time are minimized. Thus, over a long period of time, the piezoelectric oscillator can maintain high frequency accuracy.
In addition, when the temperature of the piezoelectric oscillator decreases abruptly, for example, when an apparatus incorporating the piezoelectric oscillator is moved into a lower temperature environment, condensation can occur on the circuit substrate. However, the arrangement of preferred embodiments of the present invention permits moisture content to evaporate quickly through the clearance. Thus, the influence of condensation on the circuitry is minimized.
Moreover, since the clearance is defined in the wall surrounding the cavity of the circuit substrate, it is unnecessary to reduce the area for mounting components on the circuit substrate, and the clearance is not a hindrance to the design of the wiring patterns. Also, the clearance does not hinder the miniaturization of the piezoelectric oscillator.
In the above piezoelectric oscillator, the clearance may be defined in each of two opposing walls of the cavity such that both clearances do not face each other.
In this case, since the clearances are arranged so as not to face each other, a cleaning solution can easily spread over every part of the circuit substrate. In addition, the cleaning solution can be quickly discharged after the cleaning.
Furthermore, in the piezoelectric oscillator of preferred embodiments of the present invention, each clearance may be defined on an edge of the wall.
In the piezoelectric oscillator having the clearance defined in the edge of the wall of the circuit substrate, the clearance can be easily formed when the circuit substrate is manufactured. Thus, an increase in manufacturing cost is prevented even though the advantageous clearance is provided.
Other features, characteristics, elements and advantages of the present invention will become apparent from the following detailed description of preferred embodiments with reference to the attached drawings.